This invention is directed to the production of glasses having high indices of refraction, i.e., glasses exhibiting indices of refraction higher than 1.6, preferably higher than 1.65. More particularly, this invention is directed to the production of glasses designed for use as segments (buttons) for high index multifocal photochromic lenses.
Multifocal photochromic ophthalmic products were initially marketed by Corning Glass Works, Corning, New York (now Corning Incorporated) in about the middle of 1970s. Compositions were developed for high refractive index segments which could be sealed to major lens portions (crowns) prepared from photochromic glasses marketed by Corning under the trademark PHOTOGRAY with the designations 20 8097 and 8098. Those glass compositions were included in U.S. Pat. No. 3,973,976 (Boyd) and were designed to exhibit refractive indices between 1.58-1.71, softening points between 630.degree.-675.degree. C., strain points between 450.degree.-500.degree. C., good chemical durability, and linear coefficients of thermal expansion (25.degree.-100.degree. C.) between 45-60.times.10.sup.-7 /.degree.C. Those glasses consisted essentially, in weight percent, of 3-7% B.sub.2 O.sub.3, 3-11% Al.sub.2 O.sub.3, 30-60% PbO, and 27-55% SiO.sub.2.
In the early 1980s Corning began to market extensively a different photochromic glass under the trademark PHOTOGRAY EXTRA with the designation 8111. Because the physical properties of this glass were dissimilar from those demonstrated by the earlier glasses, a different segment glass was required which displayed physical properties more closely compatible with those of the new photochromic glass than those demonstrated by the glasses of U.S. Pat. No. 3,973,976. The ensuing research led to the development of glass compositions included within U.S. Pat. No. 4,211,569 (Hares et. al.). Those glasses were designed to exhibit refractive indices between 1.58-1.71, softening points between 620.degree.-645.degree. C., strain points between 445.degree.-480.degree. C., linear coefficients of thermal expansion between 60-66.times.10.sup.-7 /.degree.C., and excellent chemical durability. Those glasses consisted essentially, in weight percent, of 0-2% Li.sub.2 O, 0-3% Na.sub.2 O, 1-4% Li.sub.2 O+Na.sub.2 O, 4-9% BaO, 25-50% PbO, 4-8% B.sub.2 O.sub.3, 30-50% SiO.sub.2, 0--3% La.sub.2 O.sub.3, 0-2% ZrO.sub.2, and 0.5-4% TiO.sub.2.
The photochromic glasses used for the major portions in multifocal products have had a refractive index of 1.523. It had been recognized that a photochromic glass exhibiting a higher refractive index would offer cosmetic improvement to eyeglasses, especially those having high minus prescriptions. The primary advantage resulting from the use of glasses demonstrating higher refractive indices resides in the fact that the required edge thickness is less. To attain comparable power for close vision, i.e., reading distance, higher refractive index segment glasses are needed for use with crown glasses having a refractive index of 1.6. As can be appreciated, the higher segment glasses must also exhibit all of the other chemical and physical characteristics demanded in conventional segment glasses; e.g., good chemical durability, coefficients of thermal expansion compatible with the crown glass, liquidus/viscosity relationships conducive for good melting and forming capabilities, low temperature viscosity values permitting good sealing to the major portion glass, etc.
To fill the requirements of multifocal ophthalmic products over the total spectrum of eyeglass prescriptions, several glasses that cover a range of refractive indices are needed. The refractive indices of button glasses that have been used to fill prescriptions with 1.523 crown glasses are recited below and are compared with those required for use with major portion glasses demonstrating a refractive index of 1.6.
______________________________________ 1.523 Crown 1.6 Crown (A) 1.6 Crown (B) ______________________________________ 1.5880 1.6750 1.6670 1.6173 1.7085 1.6950 1 6533 1.7500 1.7330 1.7013 1,8051 1,7895 ______________________________________
The 1.6 crown glasses designated (A) reflect refractive index lenses which are finished utilizing tools currently standard in the industry. The 1.6 crown glasses designated (B) reflect refractive index lenses which are more pleasing cosmetically, but which require different tools for finishing. In light of that situation, it will be necessary to be able to supply lenses from each of the eight categories to satisfy both groups of customers.
Photochromic glass lenses exhibiting a refractive index of 1.6 have been marketed by Corning under the designations 8171 and 8173. The composition of those lenses is included in U.S. application Ser. No. 07,581,789, filed Sept. 13, 1990 by Luc Grateau et. al. under the title FAST FADING, HIGH REFRACTIVE INDEX GLASS, now U.S. Pat. No. 5,023,209. That glass has a softening point of about 687.degree. C., a strain point of about 495.degree. C., and a linear coefficient of thermal expansion (25.degree.-300.degree. C.) of about 60.times.10.sup.-7 /.degree.C.
As can be observed, the segment glasses described in U.S. Pat. No. 3,973,976 and in U.S. Pat. No. 4,211,569 could be considered for use with 8171 and 8173 glasses at refractive indices up to 1.71. The objective of the present invention, however, was to devise a family of base glasses, the members of which exhibit properties rendering them suitable for sealing to 8171 and 8173 glasses, and which can exhibit refractive indices as high as about 1.81. Stated in another way, the goal of our research was to develop a range of glass compositions wherein, by varying the concentrations of the individual components, glasses could be produced displaying refractive indices across the whole spectrum of about 1.66-1.81. The efficiencies in commercial production attendant in utilizing glass compositions within a single family are self-evident.